This invention relates to controlling interwinding coupling coefficients and leakage inductances of a transformer, and use of such a transformer in a high-frequency switching circuit, such as, for example, a high frequency switching power converter.
With reference to FIG. 1, which shows a schematic representation of an electronic transformer having two windings 12, 14, the lines of flux associated with current flow in the windings will close upon themselves along a variety of paths. Some of the flux will link both windings (e.g. flux lines 16), and some will not (e.g. flux lines 20, 22, 23, 24, 26). Flux which links both windings is referred to as mutual flux; flux which links only one winding is referred to as leakage flux. The extent to which flux generated in one winding also links the other winding is expressed in terms of the winding's coupling coefficient: a coupling coefficient of unity implies perfect coupling (i.e. all of the flux which links that winding also links the other winding) and an absence of leakage flux (i.e. none of the flux which links that winding links that winding alone). From a circuit viewpoint, the effects of leakage flux are accounted for by associating an equivalent lumped value of leakage inductance with each winding. An increase in the coupling coefficient translates into a reduction in leakage inductance: as the coupling coefficient approaches unity, the leakage inductance of the winding approaches zero.
Control of leakage inductance is of importance in switching power converters, which effect transfer of power from a source to a load, via the medium of a transformer, by means of the opening and closing of one or more switching elements connected to the transformer's windings. Examples of switching power converters include DC-DC converters, switching amplifiers and cycloconverters. For example, in conventional pulse width modulated (PWM) converters, in which current in a transformer winding is interrupted by the opening and closing of one or more switching elements, and in which some or all of the energy stored in the leakage inductances is dissipated as switching losses in the switching elements, a low-leakage-inductance transformer (i.e. one in which efforts are made to reduce the leakage inductances to values which approach zero) is desired. For zero-current switching converters, in which a controlled amount of transformer leakage inductance forms part of the power train and governs various converter operating parameters (e.g. the value of characteristic time constant, the maximum output power rating of the converter; see, for example, Vinciarelli, U.S. Pat. No. 4,415,959, incorporated herein by reference), a controlled-leakage-inductance transformer (i.e. one which exhibits finite, controlled values of leakage inductance) is required. One trend in switching power conversion has been toward higher switching frequencies (i.e. the rate at which the switching elements included in a switching power converter are opened and closed). As switching frequency is increased (e.g. from 50 KHz to above 100 KHz) lower values of transformer leakage inductances are usually required to retain or improve converter performance. For example, if the transformer leakage inductances in a conventional PWM converter are fixed, then an increase in switching frequency will result in increased switching losses and an undesirable reduction in conversion efficiency (i.e. the fraction of the power drawn from the input source which is delivered to the load).
A transformer with widely separated windings has low interwinding (parasitic) capacitance, high static isolation, and is relatively simple to construct. In a conventional transformer, however, the coupling coefficients of the windings will decrease, and the leakage inductance will increase, as the windings are spaced farther apart. If, for example, a transformer is configured as shown in FIG. 1, then flux line 23, generated by winding #1, will not link winding #2 and will therefore form part of the leakage field of winding #1. If, however, winding #2 were brought closer to, or overlapped, winding #1, then flux line 23 would form part of the mutual flux linking winding #2 and this would result in an increase in the coupling coefficient and a decrease in leakage inductance. Thus, in a transformer of the kind shown in FIG. 1, the coupling coefficients and leakage inductances depend upon the spatial relationship between the windings.
Prior art techniques for controlling leakage inductance have focused on arranging the spatial relationship between windings. Maximizing coupling between windings has been achieved by physically overlapping the windings, and a variety of construction techniques (e.g. segmentation and interleaving of windings) have been described for optimizing coupling and reducing undesirable side effects (e.g. proximity effects) associated with proximate windings. In other prior art schemes, multifilar or coaxial windings have been utilized which encourage leakage flux cancellation as a consequence of the spatial relationships which exist between current carrying members which form the windings, or both the magnetic medium and the windings are formed out of a plurality of small interconnected assemblies, as in "matrix" transformers. Transformers utilizing multifilar or coaxial windings, or of matrix construction, exhibit essentially the same drawbacks as those using overlapping windings, but are even more difficult and complex to construct, especially where turns ratios other than unity are desired. Thus, prior art techniques for controlling coupling, which focus on proximity and construction of windings, sacrifice the benefits of winding separation.
It is well known that conductive shields can attenuate and alter the spatial distribution of a magnetic field. By appearing as a "shorted turn" to the component of time-varying magnetic flux which might otherwise impinge orthogonally to its surface, a conductive shield will support induced currents which will act to counteract the impinging field. Use of conductive shields around the outside of inductors and transformers is routinely used to minimize stray fields which might otherwise couple into nearby electrical assemblies. See, for example, Crepaz, Cerrino and Sommaruga, "The Reduction of the External Electromagnetic Field Produced by Reactors and Inductors for Power Electronics", ICEM, 1986. Use of an electric conductor and a cylindrical conducting ring as a means of reducing leakage fields in induction heaters are described, respectively, in Takeda, U.S. Pat. No. 4,145,591, and Miyoshi & Omori, "Reduction of Magnetic Flux Leakage From an Induction Heating Range" IEEE Transactions on Industry Applications, Vol 1A-19, No. 4, July/August 1983. British Patent Specification 990,418, published Apr. 28, 1965, illustrates how conductive shields, which form a partial turn around both the core and the windings of a transformer having tapewound windings, can be used to modify the distribution of the leakage field near the edges of the tapewound windings, thereby reducing losses caused by interaction of the leakage field with the current in the windings. Persson, U.S. Pat. No. 4,259,654, achieves a similar result by extending the width of the turn of a tapewound winding which is closest to the magnetic core.
The effects of conductive shields on the distribution of electric fields is also well known. In transformers, conductive sheets have been used as "Faraday shields" to reduce electrostatic coupling (i.e. capacitive coupling) between primary and secondary windings.